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Rising temperatures on the ‘Roof of the World’: How Tibet’s grasslands are responding to climate change

Submitted by Kristi Foster on February 6, 2013

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Kristi Foster

Sitting atop the world at over 4000 metres above sea level, the Tibetan Plateau seems a region beyond the reach of human influence. Yet the Plateau, which measures over 2.5 million square kilometers in Central Asia, is experiencing more rapid temperature increases than the global average - and its grasslands cannot adapt at such a pace.

“Climate change is creating temperature conditions that should be prolonging the growing season of grasslands, but that’s not what we’re seeing,” said Haiying Yu, a GIS Specialist with the World Agroforestry Centre’s (ICRAF’s) East-Asia Program. Yu, lead author of new research in the field explained, “What we’re seeing instead is an increase in growth rate accompanied by a shorter reproductive cycle.”

Yu was among a team of scientists from ICRAF’s Climate Change Unit that used satellite data from the Global Inventory Modeling and Mapping Studies group (GIMMS) to analyze meadow and steppe growth on the Tibetan Plateau across different stages of its seasonal life cycle.

By comparing this information with historical data on monthly temperature and precipitation, Yu and co-authors determined how grassland growth has responded to changes in climate from 1982 to 2006.

There’s no doubt that the Tibetan Plateau is heating up. Mean annual temperature has increased significantly over the 25 year span – in some cases up to 2˚ Celsius per decade, while total annual precipitation has also increased slightly.

The authors discovered the following from their analysis, against this backdrop of climatic changes:

The photosynthetic capacity of vegetation – i.e. the maximum rate at which photosynthesis can occur - has increased significantly throughout the growing season.

The start of the growing season is occurring slightly later.

Late season life cycle stages of the grasses – including (i) the time at which plants reach maturity; (ii) the time at which grass cells begin to decline; and (iii) the end of the growing season – are occurring earlier, up to two weeks per decade in some areas of the Plateau.

Overall, the growing season has shortened, despite a longer warm growing period.

Erick Okuto, a statistician with ICRAF’s Climate Change Unit, attributed many of these changes to higher temperatures during the growing season and even during the previous winter. “The start and end of grass life cycle stages are influenced by a combination of different weather conditions during different months,” he explained. “This system of cause and effect is much more complex than linking changes to specific climatic drivers.”

According to Eike Luedeling, a Climate Change Scientist with ICRAF, Tibetan grasses require exposure to a certain amount of cold winter conditions – known as winter chill – in order to begin growth in the spring. “Higher temperatures during the winter months may mean that the plants can’t fulfill their winter chilling requirements,” said Luedeling. “This could explain why we see a delay in spring phases of grasses.”

When vegetation develops in an area over time – a process known as succession – it reaches a state in which its species are in ‘equilibrium’ with the region’s environmental conditions; this is known as a climax state.

The authors suspect that vegetation on the Tibetan Plateau is not in equilibrium with the current climate, nor do they expect it to be reached anytime soon. Yu explains that it takes time for grasses to extend their habitat through migration because they usually reproduce vegetatively, rather than through seeds or spores. “It could take decades until a new equilibrium between climate and vegetation is reached, especially if the warming trend of the Tibetan Plateau persists.”

Adapting to new climatic conditions takes considerable time, and many of the world’s ecosystems will struggle to keep up with a changing climate.

Predicting how ecosystems that are not in their climax state will respond to a new climate presents a significant challenge. “If we attempt to project the impacts of climate change and tipping points on Tibetan grasslands or other ecosystems when we have vegetation and climate that don’t match, it may be difficult to get accurate results,” said Xu.

Yet these kinds of climate impact projections are essential. Grasslands influence the carbon cycle through the uptake and release of carbon during different stages of their life cycle. At the global level, changes in primary productivity can impact the carbon cycle by altering terrestrial net carbon uptake. Climate change is already affecting these processes, and is expected to exert an even greater impact on ecosystems and pastoral livelihoods in the future.

“Accurately modeling changes in ecosystems due to climate change, especially the rate at which various species can adjust to changing environments, will require a detailed understanding of species migration, dispersal and climatic responses during all seasons,” said Luedeling.